strconv: Fix unquote to handle \a \b \v \f

Those are quote codes that Go strconv.Quote might produce. And even
though Python does not use them when quoting, it too handles those quote
codes when decoding:

    In [1]: '\r'
    Out[1]: '\r'

    In [2]: '\a\b\v\f'
    Out[2]: '\x07\x08\x0b\x0c'

https://github.com/python/cpython/blob/2.7.18-0-g8d21aa21f2c/Objects/stringobject.c#L677-L688

-> Teach strconv.unquote + friends to handle them as well.

/reviewed-by @jerome
/reviewed-on nexedi/pygolang!14

golang/strconv.py

 # -*- coding: utf-8 -*-
-# Copyright (C) 2018-2020  Nexedi SA and Contributors.
+# Copyright (C) 2018-2021  Nexedi SA and Contributors.
 #                          Kirill Smelkov <kirr@nexedi.com>
 #
 # This program is free software: you can Use, Study, Modify and Redistribute
@@ -134,7 +134,7 @@ def _quote(s):
     return b'"' + b''.join(outv) + b'"'
 
 
-# unquote decodes unicode|byte string that was produced by quote.
+# unquote decodes "-quoted unicode|byte string.
 #
 # ValueError is raised if there are quoting syntax errors.
 def unquote(s):
@@ -143,7 +143,7 @@ def unquote(s):
         raise ValueError('non-empty tail after closing "')
     return us
 
-# unquote_next decodes next unicode|byte string that was produced by quote.
+# unquote_next decodes next "-quoted unicode|byte string.
 #
 # it returns -> (unquoted(s), tail-after-")
 #
@@ -192,22 +192,26 @@ def _unquote_next(s):
             s = s[2:]
             continue
 
-        if c == b't':
-            emit(b'\t')
+        # \t \n \r
+        uc = None
+        if   c == b't':  uc = b'\t'
+        elif c == b'n':  uc = b'\n'
+        elif c == b'r':  uc = b'\r'
+        # accept also \a \b \v \f that Go might produce
+        # Python also decodes those escapes even though it does not produce them:
+        # https://github.com/python/cpython/blob/2.7.18-0-g8d21aa21f2c/Objects/stringobject.c#L677-L688
+        elif c == b'a':  uc = b'\x07'
+        elif c == b'b':  uc = b'\x08'
+        elif c == b'v':  uc = b'\x0b'
+        elif c == b'f':  uc = b'\x0c'
+
+        if uc is not None:
+            emit(uc)
             s = s[2:]
             continue
 
-        if c == b'n':
-            emit(b'\n')
-            s = s[2:]
-            continue
-
-        if c == b'r':
-            emit(b'\r')
-            s = s[2:]
-            continue
-
-        if c == b'x':   # hex   XXX also handle octals?
+        # \x?? hex
+        if c == b'x':   # XXX also handle octals?
             if len(s) < 2+2:
                 raise ValueError('unexpected EOL after \\x')
 

golang/strconv_test.py

 # -*- coding: utf-8 -*-
-# Copyright (C) 2018-2019  Nexedi SA and Contributors.
+# Copyright (C) 2018-2021  Nexedi SA and Contributors.
 #                          Kirill Smelkov <kirr@nexedi.com>
 #
 # This program is free software: you can Use, Study, Modify and Redistribute
@@ -110,6 +110,21 @@ def test_quote():
         assert qq(tin) == asstr(tquoted)
 
 
+# verify that non-canonical quotation can be unquoted too.
+def test_unquote_noncanon():
+    testv = (
+        # quoted w/o "      unquoted
+        (r'\a',             "\x07"),
+        (r'\b',             "\x08"),
+        (r'\v',             "\x0b"),
+        (r'\f',             "\x0c"),
+    )
+
+    for tquoted, tunquoted in testv:
+        q = '"' + tquoted + '"'
+        assert unquote(q) == tunquoted
+
+
 def test_unquote_bad():
     testv = (
         # in            error